1. Field of the Invention
The present invention relates to a method and apparatus for tinning electronic components.
2. The Prior Art
The tinning of electronic components comprises the step of applying a hot solder coating to metallic leads or contact points of the component package. This tinning of components prior to placement on the circuit board is known in the art and is considered necessary to providing high quality and reliable connections. In the normal manufacturing cycle, a component may be stored for a substantial period of time before being utilized. During this storage period, leads can become oxidized or otherwise subject to deleterious conditions, such that when the component is later placed on the circuit board and the board assembly then soldered (normally through an automated soldering system), an oxidized lead may not solder properly thus leading to a board fault or defect. Accordingly, many manufacturers have found it advisable to solder coat the leads of the components prior to use of the component, such that when the component is later placed on the printed circuit board, the solder "reflows" and a good solder connection is assured.
To date, many different types of systems have been used for tinning the components. These range from fully automated in-line systems such as illustrated in U.S. Pat. No. 4,654,227 wherein a component is grasped by mechanical means and carried through the required processing steps, to smaller systems wherein the components are inserted into some type of tooling or fixturing and then subsequently processed. Much of the above depends upon the type of component to be tinned. Because of the problem of handling many different types of components, a large manual operation is widely used in the industry.
One particular problem encountered in the art relates to surface mounted components. With the increasing requirement for miniaturization, the industry has moved towards a surface mount configuration wherein the components are mounted on the surface of the board rather than having the leads pass through the board. Also, due to miniaturization, these surface mounted ICs generally have leads spaced closer together than is the case with through the board technology and the tinning of the leads becomes even more important. Typical of such surface mount components are those known as LCCs (leadless chip carriers) or PLCCs (plastic leaded chip carriers). In the case of LCCs, only small contact points are provided on the bottom of the IC package, while in the case of PLCCs, either a gullwing or J-lead is formed, often with a spacing of 25 mils or less between leads. These components frequently have leads extending from all four sides of a small rectangular body, thus creating problems for mechanical grasping of the component. In the past, in order to tin these components, it has been required to process one side of the component and then the opposite side.
A further problem encountered with the tinning of such types of components and particularly leadless chip carriers is to obtain an even coating on the leads or contact points (as used herein the term leads applies to all electrical contact points where the component is soldered to the board or substrate). In this respect, a conventional tinning process, either through dipping the component in a static solder pet or passing it through a wave, frequently leaves large differences in the solder thickness on the leads. Thus, for example, one lead may have a 2 mil coating while another one on the same package may have a 10 mil coating. When the component is subsequently placed on the board and passed through a conventional soldering process, good soldering connections between the lead and board are not obtained. Also, when done either manually or by automated equipment, it has been found that reproducibility of the result has not been good.